Method for manufacturing papermaking pulp

ABSTRACT

The invention relates to a method for manufacturing papermaking pulp, including: a step of refining wood, including an ozone treatment, so as to obtain a mechanical pulp; a deacidification step including placing the mechanical pulp in contact with a deacidification composition including at least one alkaline agent for more than one hour; and a bleaching step including placing the deacidified pulp in contact with a bleaching composition; wherein the pH varies by less than 0.2 point during the last 30 minutes of the deacidification step, and the pH is 6. to 10 at the end of the deacidification step.

FIELD OF THE INVENTION

The present invention relates to a paper pulp manufacturing process.

TECHNICAL BACKGROUND

Paper pulps called “mechanical pulps” or “high-yield pulps” or “wood pulps” are obtained directly from wood by a series of mechanical (defibering and refining) treatments carried out by means of grinders and/or refiners. The pulp may then undergo a bleaching phase in one or more steps.

It is known that an ozone treatment during pulp refining helps to improve the quality of the pulp produced, in particular in terms of physical properties (especially the mechanical strength of the pulp) and to reduce the energy consumption of the process.

However, the ozone treatment raises particular problems, namely a reduction in the whiteness of the pulp, a subsequent bleaching difficulty, a loss of yield relative to the wood and an increase in the chemical oxygen demand of the bleaching effluents.

An ozone treatment as mentioned above is described in document DE 2444475. Subsequent bleaching of the pulp is mentioned.

Document FR 2329794 also describes an ozone treatment.

After the treatment, the treated pulp is mixed with a lye (caustic soda) in order to stabilize the properties conferred on the pulp by the ozone treatment. The pulp is then stored and subsequently used directly without being bleached in order to manufacture paper.

A mechanical pulp ozone treatment is described in document FR 2406023. The treatment is followed by a maturing step lasting less than 30 minutes, preferably less than 10 minutes, during which the ozonated pulp remains in alkaline medium, optionally being mixed with bleaching chemicals.

In documents FR 2388933 and FR 2388934, just as in the above documents, a maturing step follows the ozonation, in the presence of a lye (caustic soda) and optionally a bleaching agent (hydrogen peroxide), for the purpose of stabilizing the pulp and optionally bleaching it at the same time. Adding caustic soda to the ozonation stage itself in order to swell the pulp is also envisaged.

Document EP 0276608 provides an illustration of an ozone treatment method. The examples in said document show that the ozone treatment causes a loss of whiteness of the pulp.

The document entitled “The effect of ozone on mechanical pulps” by N. Soteland, Can. Wood Chem. Symp. Extended Abstracts, pages 13-20 (1976) identifies a loss of whiteness problem in paper pulps obtained from ozone-treated resinous wood. The author indicates that by adjusting the pH of the ozonated pulp with caustic soda and by adding DTPA it is possible to increase the whiteness of the pulp. A loss of yield problem due to the ozonation is also mentioned.

The document entitled “Ozone treatment of mechanical pulp, Part III: influence on optical properties” by C. A. Lindholm, in the journal Paperi ja puu-Papper och Trä{umlaut over ( )} (No. 4a, 1977, pages 217-232) describes the negative effect of ozonation on the whiteness, whiteness stability and opacity of the paper pulp. Various approaches are tested to solve the problem: modification of the ozone treatment itself; caustic soda neutralization; and heating of the ozonated pulp or ozonation accompanied by a treatment with a mixture of hydrogen peroxide and caustic soda. None of these approaches is deemed to be satisfactory both as regards the mechanical strength of the pulp, and the optical properties thereof.

The document entitled “Effect of ozone on high-temperature thermomechanical pulp” by R. W. Allison, Appita Vol. 32, No. 4, pages 279-284 (1979) studies the bleaching of ozonated paper pulp (of the HTMP type) and notes the influence of the pH of the treatments after ozonation on the yield of the process: more precisely, subsequent bleaching under alkaline conditions leads to a significant loss of yield. Extraction with caustic soda for a short time of 30 minutes after the ozonation and before the hydrogen peroxide bleaching is proposed. It should be noted that with this process the observed pulp yields and/or the whiteness levels achieved are low despite the use of a very large amount of chemicals.

The document entitled “Effect of neutralization on the bonding ability of ozone-treated mechanical pulp fibers”, by C. A. Lindholm, Cellulose Chem. Technol., Vol. 17, pages 647-653 (1983) studies in detail in what way neutralization of ozonated mechanical pulp with caustic soda is liable to impart advantageous mechanical properties on said pulp.

The document entitled “Ozone in the production of softwood and hardwood high-yield pulps to save energy and improve quality” by M. Petit-Conil, C. de Choudens and T. Espilit, Nordic Pulp and Paper Research Journal, Vol. 13, No. 1, pages 16-22 (1998), studies the influence of an ozone treatment followed by hydrogen peroxide bleaching in the presence of caustic soda on pulps prepared from various types of wood essence. This document does not address the loss of yield observed during bleaching.

The document entitled “Use of ozone in mechanical pulping processes” by M. Petit-Conil, ATTP, Vol. 57, No. 2, pages 17-26 (2003) compares the effect of various ways of introducing ozone. That document too does not address the loss of yield observed during bleaching.

Document WO 2008/081078 describes the use of caustic soda and other bases during the ozone treatment itself, so as to limit the deleterious effect of the organic acids produced during ozonation on the subsequent bleaching and on corrosion.

However, the methods used in the prior art are not completely satisfactory.

In particular, there is still a need to obtain a better yield and/or achieve high whiteness levels and/or to reduce the amount of chemicals necessary for the paper pulp treatment and/or to reduce the amount of effluents generated by the bleaching.

SUMMARY OF THE INVENTION

A first subject of the invention is a process for manufacturing a paper pulp, comprising:

-   -   a wood refining step, comprising an ozone treatment, so as to         obtain a mechanical pulp;     -   a deacidification step, in which the mechanical pulp is brought         into contact with a deacidification composition comprising at         least one alkaline agent for more than 1 hour; and     -   a bleaching step in which the deacidified pulp is brought into         contact with a bleaching composition, in which the pH varies by         less than 0.2 units during the last 30 minutes of the         deacidification step and the pH is between 6 and 10 at the end         of the deacidification step.

According to one method of implementation, the deacidification step is equal to or greater than 1 hour 30 minutes, preferably equal to or greater than 2 hours and ideally equal to or greater than 3 hours.

According to one method of implementation, the pH at the end of the deacidification step is between 7 and 8.

According to one method of implementation, the pH varies by less than 0.1 units during the last 30 minutes of the deacidification step, preferably by less than 0.05 units.

According to one method of implementation, the alkaline agent of the deacidification composition:

-   -   is chosen from oxides, hydroxides, silicates and carbonates of         alkaline-earth metals and alkali metals, ammonia, aqueous         ammonia and mixtures thereof;     -   is preferably chosen from potassium hydroxide, sodium hydroxide,         magnesium hydroxide, calcium hydroxide, sodium carbonate,         magnesium carbonate, sodium silicate and mixtures thereof; and     -   is ideally magnesium hydroxide.

According to one method of implementation, the bleaching composition comprises a bleaching agent and at least one alkaline agent:

-   -   said alkaline agent being chosen from the oxides, hydroxides,         silicates and carbonates of alkaline-earth metals and alkali         metals, ammonia, aqueous ammonia and mixtures thereof;     -   said alkaline agent preferably being chosen from potassium         hydroxide, sodium hydroxide, magnesium hydroxide, calcium         hydroxide, sodium silicate, sodium carbonate, magnesium         carbonate and mixtures thereof; and     -   said alkaline agent ideally comprising sodium hydroxide and/or         magnesium hydroxide.

According to one method of implementation, the process includes a chelation step, preferably between the deacidification step and the bleaching step, in which the deacidified pulp is brought into contact with a chelation composition comprising a chelating agent, preferably ethylenediaminetetraacetic acid or one of its sodium salts or diethylenetriaminepentaacetic acid or one of its sodium salts.

According to one method of implementation, 20 kg or less but preferably less than 15 kg or less and more particularly preferably 10 kg or less of alkaline agent per tonne of mechanical pulp is used during the deacidification step.

According to one method of implementation:

-   -   5 to 100 kg, preferably 10 to 50 kg and more particularly         preferably 20 to 40 kg of bleaching agent per tonne of         mechanical pulp; and     -   5 to 100 kg but preferably 10 to 70 kg, of alkaline agent per         tonne of mechanical pulp are used during the bleaching step.

According to one method of implementation, the loss of wood yield after the bleaching step is less than or equal to 20%, preferably less than or equal to 16%, ideally less than or equal to 13% or even less than or equal to 10%.

According to one method of implementation, the whiteness of the pulp after the bleaching step is equal to or greater than 65%, preferably equal to or greater than 66.3%, ideally equal to or greater than 68% or even equal to or greater than 70%.

According to one method of implementation, the process produces effluents having a chemical oxygen demand less than or equal to 210 kg per tonne of pulp, preferably less than or equal to 180 kg per tonne of pulp, or particularly preferably less than or equal to 160 kg per tonne of pulp or even less than or equal to 145 kg per tonne of pulp.

According to one method of implementation, the mechanical pulp is an SGW, PGW, RMP, TMP, HTMP or CTMP pulp.

According to one method of implementation, the wood is a resinous wood, a deciduous tree wood or a mixture thereof, preferably a resinous wood and more particularly preferably a pinewood or spruce wood.

Another subject of the invention is a paper manufacturing process, comprising the manufacture of paper pulp according to the process described above, and the use of this paper pulp for producing paper.

The present invention makes it possible to overcome the drawbacks of the prior art. More particularly, it provides a more effective paper pulp bleaching process having a better yield and/or using a small amount of chemicals and/or generating less effluent to be treated.

This is accomplished thanks to a deacidification step between the ozone treatment and the bleaching, carried out for a long enough time until the pH is sufficiently stabilized.

Specifically, after the ozonation, the acids within the fibers tend to be progressively released. The nature and the duration of the deacidification according to the invention are therefore adapted so as to obtain a stable pH before the bleaching step, thereby making it possible to neutralize all the acid species and obtain better bleaching.

Depending on certain particular methods of implementation, the invention also has one or more of the advantageous characteristics mentioned below:

-   -   the invention makes it possible to obtain a paper pulp bleaching         efficiency equivalent to or greater than that of the prior art         (with a whiteness parameter of the same order or even higher);     -   using a mild base (especially magnesium hydroxide, calcium         hydroxide, sodium carbonate, magnesium carbonate, sodium         silicate or mixtures thereof, and preferably magnesium         hydroxide) as the alkaline agent of the deacidification         composition, the yield and/or the reduction in the amount of         chemicals necessary and the amount of effluent to be treated         and/or the whiteness are further improved;     -   also using a mild base during the bleaching step, the yield, the         reduction of the amount of chemicals necessary and the amount of         effluent to be treated, or even the whiteness, are further         improved; and     -   the mechanical properties of the pulps obtained using the         process of the invention are as satisfactory as those obtained         in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of the variation in pH during a deacidification step according to the invention (experimental measurement), in which the time in minutes is plotted on the x-axis and the pH is plotted on the y-axis.

DESCRIPTION OF METHODS OF IMPLEMENTATION OF THE INVENTION

The invention will now be described in greater detail, but in a nonlimiting manner, in the following description.

Schematically, the paper pulp manufacturing process according to the invention comprises four successive steps:

-   -   wood refining, comprising an ozone treatment, so as to obtain a         mechanical pulp;     -   deacidification, in which the mechanical pulp is brought into         contact with a deacidification composition comprising at least         one alkaline agent;     -   optionally, chelation, in which the pulp is brought into contact         with a chelation composition comprising a chelating agent; and     -   bleaching, in which the pulp is brought into contact with a         bleaching composition.

The expression “the pH varies by less than X units during the last 30 minutes of the deacidification step” means that the absolute value of the difference between the measured pH at the end of the deacidification step and the pH measured 30 minutes before the end of the deacidification step is less than X units.

The “end of the deacidification step” coincides with the start of the next step. The start of the next step may denote the start of the chelation step or the start of the bleaching step, but it may also denote the start of an intermediate washing step or the start of an intermediate change-of-consistency step. In general, “the next step” corresponds to a new action that differs from the deacidification action and comes immediately thereafter.

First step: Refining and Ozone Treatment

The wood refining and ozone treatment are carried out in the manner conventionally employed in the field.

The raw material is any kind of suitable wood, especially resinous wood, deciduous tree wood or a mixture of the two. Preferably, it is resinous wood. Examples of appropriate wood essences include pine and spruce.

Before refining, the wood may undergo one or more preliminary treatments, for example debarking, removal of wood knots, heat treatment, pressurization, cutting into chips, shives, logs or the like.

The refining comprises grinding (defibering) in a grinder (under a stream of water) or in a disk refiner.

The refining may comprise several stages. For example, after a first grinding/defibering, the product may be separated into an accepted fraction and a rejected fraction, the rejected fraction then being refined anew before being mixed with the accepted fraction. It is possible to carry out such intermediate separations several times.

During refining, an ozone treatment (ozonation) is carried out to make it easier to separate the wood fibers. The ozone treatment therefore helps to reduce the specific energy needed to separate the fibers and to increase the mechanical properties of the paper pulp thus treated. The ozone treatment is carried out according to the methods known to those skilled in the art, for example according to the methods described in the documents of the prior art mentioned in the introduction of the present application. The ozone loading is typically 0.5 to 8%, preferably 1 to 4%. The ozone treatment may relate to all of the pulp or only a fraction thereof, for example the rejected fraction from the separation after the first refining step.

Optionally, the pH may be adjusted during the ozonation or before the ozonation, for example by adding an alkaline agent, especially sodium hydroxide. Thus, it is possible to adjust the pH of the entire pulp after a first refining step or the pH of the rejected fraction after the separation, for example between 9 and 11. The reader may refer on this subject to the document WO 2008/081078.

The addition of an alkaline agent at this stage serves to minimize the drop in pH during the ozone treatment. This drop in pH results from the appearance and accumulation of acid organic by-products that are generated by the oxidizing action of ozone on the constituents of the wood. It should be noted that, even in the case in which the pH is adjusted before ozonation, the pulp is highly acid after the ozonation: the pH is between 5 and 7 immediately after the ozonation step. After the second successive ozonation refining step, the acid organic by-products are released into the stock, which has a very acid pH (generally between 3 and 4).

After the ozone refining/treatment, a mechanical pulp is obtained that may be of any of the types known to those skilled in the art, namely in particular:

-   -   a stone groundwood (SGW) pulp obtained from logs or blocks         treated at atmospheric pressure by defibrator grinders;     -   a pressure groundwood (PGW) pulp obtained from logs or blocks         treated under pressure using defibrator grinders;     -   a refiner mechanical pulp (RMP) obtained from chips or shives in         refiners working at atmospheric pressure;     -   a thermomechanical pulp (TMP) or high-temperature         thermomechanical pulp (HTMP) obtained from chips or shives in         refiners after heat treatment of the wood at high vapor         pressure; and     -   a chemithermomechanical pulp (CTMP) obtained by defibering under         pressure followed by chemical impregnation in the presence of         caustic soda and sodium bisulphite at a temperature above 100°         C.

Second Step: Deacidification

The deacidification preferably takes place after the ozone refining/treatment, that is to say once the ozone refining/treatment has been completed. It consists in neutralizing the acid by-products generated by the action of ozone on the constituents of the wood, which acid by-products accumulate within and on the outside (immediate environment) of the wood fibers. Even if a base were to be used before or during the ozonation, the deacidification involved here is in any case useful in order to neutralize the acid species, since the acids within the fibers are released after the ozonation, and the stock tends to become ever more acid over the course of time.

The deacidification comprises, preferably consists of, bringing the mechanical pulp from the first step into contact with a deacidification composition.

The contacting operation preferably takes place by simple mixing. Typically, before the deacidification the pH lies within a 3 to 5 pH range, depending on the ozone treatment (amount of ozone, optional addition of caustic soda before or during the ozonation). At the end of deacidification, the pH lies in the 6 to 12, preferably 7 to 8, range.

The deacidification composition is preferably an aqueous solution comprising an alkaline agent. This alkaline agent may be chosen from the oxides, hydroxides, silicates and carbonates of alkali metals and alkaline-earth metals, ammonia, aqueous ammonia and mixtures thereof. Among the preferred basic species to be chosen as alkaline agent, mention may be made of potassium hydroxide, sodium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, sodium silicate, magnesium carbonate and mixtures thereof.

According to a preferred method of implementation, the alkaline agent comprises a mild base. The term “mild base” is understood to mean a base that dissociates only partially in water (pKb>0) and/or has a low solubility. In particular, the alkaline agent may comprise a mild base chosen from magnesium hydroxide, calcium hydroxide, sodium carbonate, sodium silicate, magnesium carbonate and mixtures thereof, preferably magnesium hydroxide.

According to one method of implementation, the alkaline agent comprises a mild base as mentioned above in combination with another base, for example sodium (or potassium) hydroxide. However, it is preferable for the deacidification composition not to contain sodium (or potassium) hydroxide and for the alkaline agent to consist of a mild base as defined above.

Compared with sodium or potassium hydroxide, the use of a mild base such as magnesium hydroxide minimizes the extraction and dissolution of certain oxidized constituents of the wood after the ozone action. Therefore, the loss of yield is smaller and the polluting load resulting from the extraction is reduced.

The amount of alkaline agent used during the deacidification step is preferably less than or equal to 20 kg, more particularly preferably less than or equal to 15 kg and ideally less than or equal to 10 kg per tonne of mechanical pulp.

The duration of the deacidification step is preferably greater than 1 hour, more particularly preferably greater than or equal to 1 hour 30 minutes, ideally greater than or equal to 2 hours or even greater than or equal to 3 hours, so as to optimize the positive effect of this step on the total yield, on the production of effluents and on the whiteness.

The deacidification step is preferably carried out at a temperature between 4 and 95° C., more particularly preferably between 25 and 85° C. and ideally between 35 and 80° C. A temperature of about 70° C. is particularly appropriate.

The consistency during the deacidification step is preferably between 0.5 and 10%, more particularly preferably between 1 and 5% and ideally between 2 and 3%. The consistency or dryness value of the pulp corresponds to the percentage of dry matter expressed in relation to water.

The consistency is the ratio of the mass of pulp to the mass of stock which comprises the pulp, water and added chemical compounds. In other words, this is the mass concentration of pulp in the aqueous medium.

Third Step (Optional): Chelation

The chelation preferably takes place after the deacidification, i.e. once the deacidification step has been completed.

The chelation comprises, preferably consists of, bringing the mechanical pulp from the second step into contact with a chelation composition.

The chelation composition is preferably an aqueous solution comprising a chelating agent. The chelating agent may be any chemical compound conventionally used for this purpose in the field. Preferably, it is ethylenediaminetetraacetic acid or one of its sodium salts or diethylenetriaminepentaacetic acid or one of its sodium salts.

The chelating agent has a particular affinity for the metal cations present in trace amounts in the paper pulp. The objective of the chelation treatment is to neutralize these cations by sequestering them and removing them from the pulp by washing the latter. Implementing the chelation step helps to improve the performance of the hydrogen peroxide bleaching treatment.

The amount of chelating agent used in the chelation step is typically between 0.5 and 30 kg, preferably between 1 and 20 kg, more particularly preferably between 2 and 10 kg and ideally between 3 and 5 kg per tonne of mechanical pulp.

The duration of the chelation step is preferably equal to or greater than about 30 minutes.

The chelation step is preferably carried out at a temperature between 4 and 95° C., more particularly preferably between 25 and 85° C. and ideally between 35 and 80° C. A temperature of about 70° C. is particularly appropriate.

The consistency during the chelation step is preferably between 0.5 and 10%, more particularly preferably between 1 and 5% and ideally between 2 and 3%.

It should be noted that the chelation step may also be carried out at the same time as the deacidification (and not thereafter) in order to limit the number and volume of the equipment items necessary for implementing the process. In this case, all that is required is to bring the pulp into contact with the chelating agent at the deacidification stage, either by adding the latter separately or by including it in the deacidification composition, preferably in the amounts indicated above.

Fourth Step: Bleaching

The bleaching is preferably carried out after the chelation (or after the deacidification if there is no chelation), i.e. once the chelation step (or the deacidification step if there is no chelation) has been completed.

The bleaching comprises, preferably consists of, bringing the mechanical pulp from the third step into contact with a bleaching composition.

The consistency during the bleaching step is preferably between 5 and 50%, more particularly preferably between 10 and 45% and ideally between 20 and 40%.

The bleaching rate is more rapid at high consistency (whereas the chelation rate is rapid even at low consistency). The consistency of the pulp may be increased, for example by pressing it and eliminating filtrates, comprising especially chelated metals.

Preferably, the contacting step takes place by the bleaching composition being simply mixed with the pulp. The type of apparatus used for the mixing operation is adapted according to the consistency of the pulp: direct mixing by means of an injection pump if the consistency is low or moderate (less than 10%), while a mixer is used for higher consistency (up to about 40%).

The bleaching composition is preferably an aqueous solution comprising a bleaching agent and an alkaline agent.

The bleaching agent may be any chemical compound conventionally used for this purpose in the field. Preferably, it is hydrogen peroxide, but sodium hydrosulphite may also be used.

According to one method of implementation, the alkaline agent may comprise (or consist of) a mild base as defined in the second step.

According to one method of implementation, the alkaline agent may comprise (or consist of) a base different from a mild base as defined in the second step, for example it may comprise (or consist of) sodium (or potassium) hydroxide.

According to a preferred method of implementation, the alkaline agent may comprise (or consist of) a mixture of a mild base, as defined in the second step, and a base different from a mild base (for example sodium or potassium hydroxide). A mixture of sodium hydroxide and magnesium hydroxide constitutes a preferred alkaline agent in the bleaching composition.

In this case, according to one method of implementation, the ratio by weight of the mild base to the base different from a mild base (for example the ratio by weight of sodium hydroxide to magnesium hydroxide) is between 0.001 and 1000, preferably between 0.01 and 100, more particularly preferably between 0.1 and 10 and ideally between 0.2 and 5.

The presence of a mild base in the bleaching composition minimizes the extraction and dissolution of certain oxidized constituents of the wood after the action of the ozone. As a consequence, the use of a mild base makes it possible to further improve the wood yield, to reduce the effluents and the consumption of chemicals or even to improve the whiteness of the pulp. In practice, the ratio by weight of the mild base to the base different from a mild base generally results from a compromise between whiteness and yield.

Preferably, the alkaline agent of the bleaching composition comprises sodium silicate. Sodium silicate has the additional function of stabilizing the bleaching agent (especially hydrogen peroxide). It is also possible to provide in the bleaching composition another stabilizing agent in addition to or in place of the sodium silicate. Polyhydroxyacrylate-type compounds constitute possible stabilizing agents.

The amount of bleaching agent used is typically between 5 and 100 kg, preferably between 10 and 50 kg and more particularly preferably between 20 and 40 kg per tonne of mechanical pulp.

The amount of alkaline agent used is typically between 5 and 100 kg, preferably between 10 and 70 kg per tonne of mechanical pulp.

The bleaching composition may also comprise a chelating agent as defined above, especially if the chelation step is absent or has resulted in incomplete chelation.

It should be noted that the bleaching composition may be prepared separately and then brought into contact with the pulp, but it may also be prepared directly upon contact with the pulp. In the second case, the various compounds of the bleaching composition are added in succession directly to the pulp.

The duration of the bleaching step varies depending on the type of agent used.

In the case of hydrogen peroxide, this duration is typically between 10 minutes and 8 hours, preferably between 30 minutes and 6 hours and more particularly preferably between 2 hours and 4 hours.

The bleaching step is preferably carried out at a temperature between 4 and 95° C., more particularly preferably between 25 and 85° C. and ideally between 35 and 80° C. A temperature of about 70° C. is particularly appropriate.

After the process, a paper pulp is obtained that preferably has one or more of the following characteristics:

-   -   the whiteness is equal to or greater than 65%, preferably equal         or greater than 66.5%, ideally equal to or greater than 68% or         even equal to or greater than 70%;     -   the loss of wood yield of the process is less than or equal to         20%, preferably less than or equal to 16%, ideally less than or         equal to 13% or even less than or equal to 10%;     -   the chemical oxygen demand (COD) of the effluents is less than         or equal to 210 kg per tonne of pulp, preferably less than or         equal to 180 kg per tonne of pulp, more particularly preferably         less than or equal to 160 kg per tonne of pulp or even less than         or equal to 145 kg per tonne of pulp.

In the context of the present application, the whiteness of the pulp is defined according to the ISO 2470 standard.

In the context of the present invention, the loss of wood yield of the process is defined as follows: after each step (deacidification, chelation, bleaching), a 25% fraction of the aqueous filtrate is recovered by pressing the stock. The water is evaporated by heating the filtrate at 50° C. until a dry residue of constant mass (corresponding to the residue dissolved in the filtrate and therefore lost) is obtained. The loss of yield of the step in question is calculated in the following manner:

Loss of yield step=(100/25)×(mass of residue at the end of the step)/mass of initial pulp [after the refining step].

The loss of yield of the process is the sum of the losses of yield observed for each of the deacidification (when implemented), chelation and bleaching steps. This loss of yield does not take into account the ozonation step.

In the context of the present application, the chemical oxygen demand of the effluents is defined according to the ISO 15705 standard, the measurement being carried out using a Hach DR/2000 spectrophotometer.

EXAMPLES

The following examples illustrate the invention, but without limiting it.

Example 1

A pine pulp was prepared using the TMP process. The pulp underwent an ozone treatment during the refining phase. More precisely, the reject from the primary refiner was treated with 2% ozone, the pH being adjusted by adding 5 kg of caustic soda per tonne of pulp, and then the ozone-treated reject was mixed with the accepted fraction from the secondary refiner. The whiteness of the pulp after the ozone refining/treatment was 47.3%.

After the ozone refining/treatment, a chelation step was provided, during which the pulp was treated with 4 kg of DTPA per tonne of pulp, with a consistency of 4% and at a temperature of 70° C. and a pH of 6-7 for 1 hour.

After the chelation step, the pulp underwent a bleaching step, with a consistency of 20%, at a temperature of 70° C. for a time of 2 hours.

To prepare pulp A (comparative example), the following parameters were also used:

-   -   no deacidification step between the ozone refining/treatment and         the chelation; and     -   bleaching composition:         -   hydrogen peroxide: sufficient amount for 30 kg/tonne of             pulp,         -   sodium silicate: sufficient amount for 30 kg/tonne of pulp             and         -   sodium hydroxide: sufficient amount for 42.5 kg/tonne of             pulp.

To prepare pulp B, the following parameters were also used:

-   -   a deacidification step lasting 3 hours between the ozone         refining/treatment and the chelation, by means of kg of sodium         hydroxide per tonne of pulp (7.3 pH at the end of the         treatment); and     -   a bleaching composition identical to that used for pulp A except         for the amount of sodium hydroxide: 32.5 kg per tonne of pulp         instead of 42.5 kg.

To prepare pulp C, the following parameters were also used:

-   -   a deacidification step lasting 3 hours between the ozone         refining/treatment and the chelation, by means of 7 kg of         magnesium hydroxide per tonne of pulp (7.2 pH at the end of the         treatment); and     -   a bleaching composition identical to that used for pulp B.

To prepare pulp D, the following parameters were also used:

-   -   a deacidification step identical to that used for pulp C (7.2 pH         at the end of the treatment); and     -   a bleaching composition identical to that used for pulp A except         for the amount of sodium hydroxide (8 kg instead of 42.5 kg per         tonne of pulp) and the addition of 13.5 kg of magnesium         hydroxide per tonne of pulp.

The whiteness of the pulp at the end of the process, the loss of yield relative to the initial wood and the total chemical oxygen demand (COD) generated were measured on each pulp. The results are given Table 1 below.

TABLE 1 Parameters measured on pulps A to D COD per tonne of Pulp Whiteness Loss of yield pulp A  68% 13.2% 185 kg B 70.7% 12.7% 178 kg C 70.2% 12.2% 171 kg D 65.9%  10% 140 kg

The comparison between pulp A on the one hand and pulp B, C and D on the other, shows that the presence of the deacidification step improves the whiteness of the pulp and the wood yield of the process and reduces the COD.

The comparison between pulp B and pulp C shows that the use of magnesium hydroxide in place of sodium hydroxide during the deacidification step, with a reduction of 30% in the amount of chemical necessary, improves the wood yield of the process and reduces the COD, with only a slightly lower whiteness.

The comparison between pulp C and pulp D shows that a reduction of about 75% in the mass of sodium hydroxide used in the bleaching step, and replacing about 41% of this sodium hydroxide with magnesium hydroxide enables the loss of yield and the COB to be reduced by 20%.

As regards the manufacture of pulp B, an experiment was also carried out to measure the pH during the deacidification step, by prolonging the latter beyond 3 hours. The result of this experiment is given in Table 2 below, and also in FIG. 1. Time t=0 corresponds to the start of the deacidification.

TABLE 2 Change in pH during the deacidification step (pulp B) Time (in min) pH 0 4.06 1 10.25 4 9.93 9 9.33 19 8.59 30 8.27 60 7.81 90 7.56 120 7.37 150 7.28 220 7.27 260 7.25

From this example it may be seen that the initially acid pH becomes highly basic in the first minute of deacidification. Subsequently, the progressive release of acids by the fibers leads to a slow reduction in the pH, which stabilizes only after 2 to 3 hours.

Example 2

A pine pulp was prepared using the TMP process. The pulp underwent an ozone treatment during the refining phase. More precisely, the reject from the primary refiner was treated with 2% ozone, the pH not being adjusted, and then the ozone-treated reject was mixed with the accepted fraction from the secondary refiner. The whiteness of the pulp after the ozone refining/treatment was 48.3%.

After the ozone refining/treatment, a chelation step was provided, during which the pulp was treated with 4 kg of DTPA per tonne of pulp, with a consistency of 2-3% and at a temperature of 70° C. and a pH of 6-7 for 1 hour.

After the chelation step, the pulp underwent a bleaching step, with a consistency of 20%, at a temperature of 70° C. for a time of 2 hours.

To prepare pulp E, the following parameters were also used:

-   -   a deacidification step lasting 3 hours between the ozone         refining/treatment and the chelation, by means of 20 kg of         sodium hydroxide per tonne of pulp (7.4 pH at the end of the         treatment); and     -   bleaching composition:         -   hydrogen peroxide: sufficient amount for 30 kg/tonne of             pulp,         -   sodium silicate: sufficient amount for 30 kg/tonne or pulp             and         -   sodium hydroxide: sufficient amount for 22.5 kg/tonne of             pulp.

To prepare pulp F, the following parameters were also used:

-   -   a deacidification step lasting 3 hours between the ozone         refining/treatment and the chelation, by means of 9 kg of         magnesium hydroxide per tonne of pulp (7.3 pH at the end of the         treatment); and     -   a bleaching composition identical to that used for pulp E.

To prepare pulp G, the following parameters were also used:

-   -   a deacidification step lasting 3 hours between the ozone         refining/treatment and the chelation, by means of 12 kg of         magnesium hydroxide per tonne of pulp (7. 6 pH at the end of the         treatment); and     -   a bleaching composition identical to that used for pulp E.

To prepare pulp H, the following parameters were also used:

-   -   a deacidification step identical to that used for pulp G (7.6 pH         at the end of the treatment); and     -   a bleaching composition identical to that used for pulp E except         for the amount of sodium hydroxide (5.6 kg per tonne of pulp         instead of 22.5 kg) and the addition of 9 kg of magnesium         hydroxide per tonne of pulp.

The whiteness of the pulp at the end of the process, the loss of yield relative to the initial wood and the total chemical oxygen demand (COD) generated were measured on each pulp. The results are given in Table 3 below.

TABLE 3 Parameters measured on pulp E to H COD per tonne of Pulp Whiteness Loss of yield pulp E 68.0% 15.9% 222 kg F 68.1% 15.1% 212 kg G 68.2% 14.6% 205 kg H 68.4% 12.8% 179 kg

Comparing pulp E on the one hand and pulp F and G on the other shows that the use of magnesium hydroxide in place of sodium hydroxide, with a reduction of 55% or 40% in the amount of chemical necessary, improves the wood yield of the process and markedly reduces the COD, but also slightly improves the whiteness.

Comparing pulp G with pulp H shows that the approximately 75% reduction in the weight of caustic soda used in the bleaching step and the replacement of about 40% of the caustic soda with magnesium hydroxide also markedly improves all the parameters of interest, namely the whiteness of the pulp, the yield and the COD.

Example 3

A spruce pulp was prepared using the TMP process. The pulp underwent an ozone treatment during the refining phase. More precisely, the reject from the primary refiner was treated with 2% ozone, the pH not being adjusted, and then the ozone-treated reject was mixed with the accepted fraction from the secondary refiner. The whiteness of the pulp after the ozone refining/treatment was 51.2%.

After the ozone refining/treatment, a chelation step was provided, during which the pulp was treated with 4 kg of DTPA per tonne of pulp, with a consistency of 2-3% and at a temperature of 70° C. and a pH of 6-7 for 1 hour.

After the chelation step, the pulp underwent a bleaching step, with a consistency of 20%, at a temperature of 70° C. and for a time of 2 hours.

To prepare pulp I (comparative example) the following parameters were also used:

-   -   a deacidification step lasting 5 to 10 minutes between the ozone         refining/treatment and the chelation, by means of 10 kg of         sodium hydroxide per tonne of pulp; and     -   bleaching composition:         -   hydrogen peroxide: sufficient amount for 30 kg/tonne of             pulp,         -   sodium silicate: sufficient amount for 30 kg/tonne of pulp             and         -   sodium hydroxide: sufficient amount for 27.5 kg/tonne of             pulp.

To prepare pulp J, the following parameters were also used:

-   -   a deacidification step identical to that for pulp I, except that         it lasted 3 hours; and     -   a bleaching composition identical to that used for pulp I,         except that the amount of sodium hydroxide used was 22.5         kg/tonne of pulp.

To prepare pulp K (comparative example) the same parameters as for pulp I were used, except that there was no deacidification step and the amount of sodium hydroxide used in the bleaching composition was 37.5 kg/tonne of pulp.

To prepare pulp L (comparative example), the same parameters as for pulp J were used, except that the duration of the deacidification step was only 30 minutes.

To prepare pulp M, the same parameters as for pulp J were used, except that the duration of the deacidification step was 90 minutes.

The whiteness of the pulp at the end of the process, the loss of yield in relation to the initial wood and the total chemical oxygen demand (COD) generated were measured on each pulp. The results are given in Table 4 below.

TABLE 4 Parameters measured on pulps I to M COD per tonne of Pulp Whiteness Loss of yield pulp I 72.4% 9.6% 134 kg J 73.0% 9.2% 128 kg K  70% 9.6% 134 kg L 71.2% 9.1% 127 kg M 72.7% 9.2% 128 kg

Comparing pulp I or pulp K on the one hand with pulp J or pulp M on the other shows that by extending the duration of the deacidification step sufficiently to stabilize the pH it is possible to improve the final whiteness while reducing the amount of reagents used in the bleaching step. In contrast, the test carried out on pulp L, compared with the tests on pulps J and M, demonstrate that, for the same amount of reactants, a relatively short deacidification step (lasting 30 minutes) gives a lower whiteness than a long deacidification step (lasting 90 minutes or more) for a similar loss of yield. 

1. A process for manufacturing a paper pulp, comprising: refining wood, comprising an ozone treatment, so as to obtain a mechanical pulp; contacting the mechanical with a deacidification composition comprising at least one alkaline agent for more than 1 hour to form a deacidified pulp; and contacting the deacidified pulp with a bleaching composition, wherein the improvement comprises the pH varies by less than 0.2 units during the last 30 minutes of the contacting with the deacidification composition and the pH is between 6 and 10 at the end of the contacting with the deacidification composition.
 2. The process as claimed in claim 1, in which the duration of the contacting with the deacidification composition is equal to or greater than 1 hour 30 minutes.
 3. The process as claimed in claim 1, in which the pH at the end of the contacting with deacidification composition is between 7 and
 8. 4. The process as claimed in claim 1, in which the pH varies by less than 0.1 units during the last 30 minutes of the contacting with the deacidification composition.
 5. The process as claimed in claim 1, in which the alkaline agent of the deacidification composition: is selected from the group consisting of oxides, hydroxides, silicates and carbonates of alkaline-earth metals and alkali metals, ammonia, aqueous ammonia and mixtures thereof.
 6. The process as claimed in claim 1, in which the bleaching composition comprises a bleaching agent and at least one alkaline agent selected from the group consisting of oxides, hydroxides, silicates and carbonates of alkaline-earth metals and alkali metals, ammonia, aqueous ammonia and mixtures thereof.
 7. The process as claimed in claim 1, which further comprisies contacting the deacidified pulp with a chelation composition comprising a chelating agent.
 8. The process as claimed in claim 1, in which 20 kg or less of alkaline agent per tonne of mechanical pulp is used.
 9. The process as claimed in claim 1, in which the bleaching composition comprises: 5 to 100 kg of bleaching agent per tonne of mechanical pulp; and 5 to 100 kg of alkaline agent per tonne of mechanical pulp.
 10. The process as claimed in claim 1, in which the loss of wood yield after contacting with the bleaching composition is less than or equal to 20%.
 11. The process as claimed in claim 1, in which the whiteness of the pulp after contacting with the bleaching composition is equal to or greater than
 65. 12. The process as claimed in claim 1, producing effluents having a chemical oxygen demand less than or equal to 210 kg per tonne of pulp.
 13. The process as claimed in claim 1, in which the mechanical pulp is selected from the group consisting of SGW, PGW, RMP, TMP, HTMP and CTMP pulp.
 14. The process as claimed in claim 1, in which the wood is selected from the group consisting of resinous wood, deciduous tree wood and a mixture thereof, preferably a resinous wood and more particularly preferably a pinewood or spruce wood.
 15. (canceled)
 16. The process as claimed in claim 1, in which the duration of the contacting with the deacidification composition is equal to or greater than 3 hours.
 17. The process as claimed in claim 1, in which the pH varies by less than 0.05 units during the last 30 minutes of the contacting with the deacidification composition.
 18. The process as claimed in claim 1, in which the alkaline agent of the deacidification composition is selected from the group consisting of potassium hydroxide, sodium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, magnesium carbonate, sodium silicate and mixtures thereof.
 19. The process as claimed in claim 1, in which the bleaching composition comprises a bleaching agent and at least one alkaline agent selected from the group consisting of sodium hydroxide, magnesium hydroxide and mixtures thereof.
 20. The process of claim 7 wherein said chelating agen is selected from the group consisting of ethylenediaminetetraacetic acid and one of its sodium salts and diethylenetriaminepentaacetic acid and one of its sodium salts.
 21. The process as claimed in claim 1, in which the bleaching composition comprises: 20 to 40 kg of bleaching agent per tonne of mechanical pulp; and 10 to 70 kg, of alkaline agent per tonne of mechanical pulp.
 22. The process as claimed in claim 1, in which the loss of wood yield after contacting with the bleaching composition is less than or equal to 10%.
 23. The process as claimed in claim 1, in which the whiteness of the pulp after the contacting with the bleaching composition is equal to or greater than 70%.
 24. The process as claimed claim 1, producing effluents having a chemical oxygen demand less than or equal to 145 kg per tonne of pulp. 